Positive electrode active material including impurity elements in a crystal defect or a cavity
Provided is a positive electrode active material for a lithium ion secondary battery having favorable cycle characteristics and high capacity. A covering layer containing aluminum and a covering layer containing magnesium are provided on a superficial portion of the positive electrode active material. The covering layer containing magnesium exists in a region closer to a particle surface than the covering layer containing aluminum is. The covering layer containing aluminum can be formed by a sol-gel method using an aluminum alkoxide. The covering layer containing magnesium can be formed as follows: magnesium and fluorine are mixed as a starting material and then subjected to heating after the sol-gel step, so that magnesium is segregated.
1. A lithium-ion secondary battery comprising:
a positive electrode comprising a positive electrode active material,
wherein the positive electrode active material comprises a first region, a second region, and a third region,
wherein the first region comprises a layered rock-salt crystal structure,
wherein the third region comprises a rock-salt crystal structure,
wherein the second region covers at least a part of the first region,
wherein the third region covers at least a part of the second region,
wherein at least the part of the second region covered by the third region comprises a layered rock-salt crystal structure,
wherein the first region comprises lithium cobaltate,
wherein the second region comprises aluminum and oxygen,
wherein the third region comprises magnesium and oxygen,
wherein a crystal orientation of the rock-salt crystal structure of the third region is substantially aligned with a crystal orientation of the layered rock-salt crystal structure of at least the part of the second region covered by the third region,
wherein the positive electrode active material includes a crystal defect in the positive electrode active material, the crystal defect being observable from a TEM image,
wherein the crystal defect includes magnesium, and
wherein a distribution of aluminum comprises a region existing deeper from a surface of the positive electrode active material than a distribution of magnesium, and
wherein the positive electrode active material is free of at least one of intentionally added nickel and manganese.
2. The lithium-ion secondary battery according to claim 1 , wherein the crystal defect exists in the first region of the positive electrode active material.
3. The lithium-ion secondary battery according to claim 1 , wherein a maximum peak of a concentration of the magnesium is positioned closer to a surface of the positive electrode active material than a maximum peak of a concentration of the aluminum in line analysis of energy dispersive X-ray spectrometry.
4. The lithium-ion secondary battery according to claim 1 , wherein the crystal defect includes aluminum.
5. A lithium-ion secondary battery comprising:
a positive electrode comprising a positive electrode active material,
wherein the positive electrode active material comprises a first region, a second region, and a third region,
wherein the first region comprises a layered rock-salt crystal structure,
wherein the third region comprises a rock-salt crystal structure,
wherein the second region covers at least a part of the first region,
wherein the third region covers at least a part of the second region,
wherein at least the part of the second region covered by the third region comprises a layered rock-salt crystal structure,
wherein the first region comprises lithium cobaltate,
wherein the second region comprises aluminum and oxygen,
wherein the third region comprises magnesium and oxygen,
wherein a crystal orientation of the rock-salt crystal structure of the third region is substantially aligned with a crystal orientation of the layered rock-salt crystal structure of at least the part of the second region covered by the third region,
wherein the positive electrode active material includes a crystal defect in the positive electrode active material, the crystal defect being observable from a TEM image,
wherein the positive electrode active material is free of at least one of intentionally added nickel and manganese,
wherein a distribution of aluminum comprises a region existing deeper from a surface of the positive electrode active material than a distribution of magnesium, and
wherein a depth at which a concentration of the magnesium becomes ⅕ of a peak of the concentration of the magnesium is 2 nm to 5 nm from the surface of the positive electrode active material in line analysis of energy dispersive X-ray spectrometry.